1.2 Discharge Relationships With Drainage Basins Flashcards
What are hydrographs? What are annual hydrographs?
These show how discharge changes over time, either annually or during a storm.
Annual hydrographs, also known as a regime are often used to study annual changes and responses to a rivers environment. They can be used to predict/monitor floodplains.
The most important factor on an annual basis is the climate with clear periods of high/low rainfall.
It is influenced by precipitation, local rocks, slopes and aspects, vegetation and soil.
What are storm hydrographs?
These depict the discharge-time relationship for an individual storm, giving it a bell shape. They also show the rainfall in mm in a bar chart to the left side, and how discharge reacts to this rainfall to the right of it.
The peak rainfall is the highest amount of rainfall.
The rising limb is how fast the discharge in the river rises, usually after the peak flow.
The peak discharge is the highest amount of discharge within the river during the storm. This indicates the size of the flood.
The recession limb shows how fast the river returns to its previous state.
The lag time is the difference between peak discharge and peak rainfall
Sometimes, the base flow is indicated which is the discharge before the storm. The storm flow as such is then the baseflow + overland flow + throughflow and so the amount from the storm is the storm flow - baseflow.
What influences hydrographs?
Duration of rainfall: more surface runoff and rain, less infiltration as it becomes saturated
Vegetation cover: higher surface runoff throughout shower, less infiltration, high interception and EVT, vegetated areas tend to have low flood peaks. More interception loss in summer.
Antecedent moisture - high surface runoff, less infiltration
Size: higher rain size means high surface runoff and less infiltration
Soil porosity: surface runoff decreases with porosity and infiltration increases - impermeable rocks have high surface runoff
Aspect: steeper slopes have high surface runoff and less infiltration, quicker floods
Climate: high evaporation so precipitation, thawing of ice, time of year. High moisture air leads to high peaks.
Urbanisation/land use: drainage channels, concrete, land uses may reduce vegetation or permeability have high peaks, farmland and ploughing can increase permeability and cause more infiltration, however reduced vegetation, drainage channels and ditches and impermeable lands may lead to reduced interception and higher surface runoffs.
Basin shape: bigger size has more rainfall so big areas may take longer or if fast flowing causes bigger floods
Drainage density: sewers and drains increase speeds and reduce lag times
What is the water budget?
The balance between precipitation and evapotranspiration. It is used to manage the water supply in a basin.
Calculated by: Soil storage = precipitation - channel flow out - evapotranspiration
Factors affecting discharge:
Climate - responds to changes in input of precipitation and outputs of evapotranspiration
Precipitation - High, intense rainfall increases discharge. Intensity means faster surface runoff. Sudden melting off snowfall can cause lots of overland flow which may also coincide rainfall due to warm water.
Antecedent - if already moist less needed to saturate
Temperature - below freezing soil is frozen so very impermeable. In high temp evapotranspiration is high
Seasonality: in temperate areas it is even across the year but less leaves in the winter may lead to a rise. In tropical areas there is high rainfall in the summer and so high discharge.
What are the characteristics of the drainage basin and how do they effect discharge?
Size: small basins have less rain, low discharge and faster response and vise versa.
Circular basins respond faster and have higher discharge
Drainage: low infiltration high drainage areas respond faster
Soil/Rock: impermeable surfaces have high surface runoff, are fast and have high discharge and vise versa. Permeable rock causes faster base flow as water can flow through spaces
Slopes: steep slopes have high surface runoff and discharge, and vise versa.
Vegetation: forests have less discharge, interception and evapotranspiration
Land use: if creates impermeable surfaces or reduces vegetation cover then surface runoff rises. Pasture land absorbs more but there is less EVT so higher discharge. Floodplains are fertile and squashed by farming so the soil is poor quality, causing less infiltration and more surface runoff.
What is the recurrence interval?
How often/likely a flood of specific magnitude is going to occur. Recorded in two ways
Number of years on record+1/Ranking of flood
Number of events/Magnitude over time
